In nanowire based devices, such as nanowire based light emitting diodes (LEDs), a multitude of nanowire based structures are usually arranged in ordered arrays on a substrate. The substrate often has multiple purposes, i.e. being a template for nanowire growth, being a carrier for the nanowires in the device and electrically connecting the nanowires on one side thereof. Different techniques for growth of the ordered arrays of nanowire based structures, where all structures are parallel and oriented in the same direction, are known. For example, semiconductor nanowires may be epitaxially grown on a high quality, mono-crystalline, semiconductor layer of the substrate, typically by selective area growth with a patterned growth mask arranged on the substrate, as described in e.g. WO 2007/102781. Another common method is the so called VLS (vapour-liquid-solid) technique where a pattern of catalytic particles, often Au, is used as seeds to grow the nanowires, as described in U.S. Pat. No. 7,335,908.
Nitride semiconductors, such as GaN InN and MN and their GaInN, GaAlN, and GaInAlN combinations of various composition are used in blue, green and UV LEDs and other optoelectronic applications due to their wide and direct bandgap. Typically, in these devices nitride semiconductors are grown in planar layers on a substrate. However, mismatch between substrate and the nitride semiconductors, for example lattice mismatch, introduce detrimental defects such as cracks in the grown material. In prior art, dislocations have been suppressed by using epitaxial substrates or substrates with an epitaxial buffer layer. Commercial GaN based devices utilize sapphire, Si or SiC substrates, which, are highly lattice mismatched to GaN, and hence several μm thick buffer layers are epitaxially grown on the substrates in order to function as a strain accommodating layers and a high quality epitaxial foundation to grow the device on. Examples of the use of epitaxial buffer layers can be found in the following documents.
U.S. Pat. No. 6,523,188 B2 discloses epitaxial growth of an epitaxial buffer layer made of AlN on a Si (111) substrate before growing the GaN layer to compensate for the large lattice mismatch between GaN and Si. The epitaxial buffer layer is preferably less than 0.2 μm in order to obtain a flat GaN layer.
In U.S. Pat. No. 6,818,061 B2 discloses epitaxial growth of a thin epitaxial buffer layer including AlN with a thickness of about 40 nm on a Si (111) substrate before growing the GaN layer to compensate for the large lattice mismatch between GaN and Si. Moreover, the GaN layer includes interlayers with alternating AlN and GaN layers.
In U.S. Pat. No. 6,617,060 B2 it is disclosed that a compositionally graded transition layer made of a GaN alloy between a Si substrate and a GaN layer, and optionally additionally a thin epitaxial strain accommodating layer that generally has a constant composition throughout its thickness, can be used to prevent crack formation in the GaN layer. Without this compositionally graded transition layer cracks can not readily be prevented.
U.S. Pat. No. 7,365,374 B2 discloses the use of a strain absorbing layer on a substrate. The strain absorbing layer should have a thickness of less than 10 nm so that overlying layers have an epitaxial relation ship with the underlying substrate.
It is appreciated from the above examples that in prior art methods the buffer layer is grown by epitaxial growth methods in order to form thick single crystalline, high quality, epitaxial buffer layers. Together with the GaN device layer grown on the epitaxial buffer layer an epitaxial layer with a thickness of more than 3 μm is formed.
Recently the use of nitride semiconductors for nanowire based devices has received considerable attention since the nanowires enable growth of nitride semiconductor materials with low defect density, as described in WO 2008/085129 A1. However, even if nanowires are used, the growth of high-quality nitride semiconductors, for example GaN, is performed using high-quality epitaxial layers as templates. The use of epitaxial layers of high quality ensures an optimal epitaxial template for nanowire growth, minimizes the density of defects that may continue up in the nanowires, and enables low electrical resistance between the substrate and nanowires. However, a buffer layer in accordance with prior art introduces substrate bowing due to the strain, which radically alters the thermal profile over the substrate. For nanowire growth, high thermal uniformity on the substrate during growth is crucial for fabrication of nanowire structures such as LEDs. The problem of substrate bowing is enhanced by increasing size of the substrate, in this way being an obstacle for large-scale processing of GaN devices on large substrates. Growth of the buffer layer is a time consuming procedure and often thick AlN is used in the buffer layer, which limits the vertical conductivity. Moreover, for many optoelectronic devices, for example LEDs the substrate is often removed, leaving only the buffer layer in the final device, whereby the costly substrate material only is used for the growth step.
In LEDs, reflectors may be used under the light emitting region to direct light out from the LED. Most common is the use of metal reflectors, as Ag mirrors. Bragg reflectors comprise repeated epitaxial semiconductor layers with different refraction index forming a. Bragg reflectors are limited in their reflectivity over a narrow span of wavelength and incident light angle and are not suitable for devices emitting light in a wider wavelength region. The narrow optimal wavelength window is less of a problem as LEDs do emit light of narrow wavelength. However, growth of Bragg reflectors is time consuming, and particularly challenging on lattice mismatched substrates, since the crystal quality has to be high to make efficient reflectors.